A vertical cavity surface emitting laser (VCSEL) has been widely used as a light source for data communications such as Ethernet (registered trademark) and Fibre Channel due to its low cost and low power consumption properties. These VCSELs have been required to provide such modulation speeds as to support high speed modulation operations at around 10 Gbps (Giga bits per second) in response to an increase in data communication capacity in recent years. Also, the VCSEL is expected as a light source for optical interconnection as well, and devices are increasingly required to support parallelism and ultra-high speeds (20 Gbps/channel).
For modulating the VCSEL at high speeds, it is necessary to increase the relaxation oscillation frequency (fr) and reduce the CR time constant. With respect to the CR time constant, a VCSEL exceeding 20 Gbps can now be fabricated by technologies which involves implantation of ions, embedding of insulating materials, and the like, for reducing the electric resistance of the VCSEL and the electric capacitance around a light emitting region. With respect to fr, on the other hand, fr can be generally increased by increasing the injection current to increase photon density. However, as a current is injected into an active layer, the temperature around a light emitting region of the VCSEL becomes very high as compared with ambient temperature, so that this constitutes one of the significant factors which dictate the upper limit of fr of the VCSEL.
In the VCSEL, a volume in which light exists is reduced to increase fr, such that high photon density can be attained even with the same current. One method therefore is to decrease an oxide aperture diameter in an oxide current confinement structure which has a large optical confinement effect. In this structure, an AlGaAs layer having a high Al composition is oxidized from a mesa side with water vapor to change part of the AlGaAs layer to an insulating material of AlOx.
This structure not only provides a current confinement structure, but also has a large optical confinement effect resulting from the difference in the refraction index between a semiconductor and an insulating material, so that as the oxide aperture diameter is reduced, the area of a portion in which light exists in plane, and the volume of the portion in which light exists, when considering a growth direction as well, are also reduced, to attain high photon density with a small current. Consequently, the VCSEL provides very high current modulation efficiency, so that high fr can be achieved before heat generation caused by current injection can exert a noticeable effect.
While the foregoing technique reduces the volume in which light exists in a VCSEL in-plane direction, the current modulation efficiency at fr can be improved by reducing the volume in which light exists in a growth direction as well, even with the same gain of the active layer.
The structure of the VCSEL in the growth direction comprises an upper and a lower DBR (Distributed Bragg Reflector), and an optical resonator unit sandwiched there between, and a light intensity forms a standing wave in the vertical direction. When the light intensity is designed to be the highest in the active layer, the resulting VCSEL characteristics exhibit a low threshold and a high efficiency. The light intensity in the DBR becomes lower at a site further away from the optical resonator unit, and an “anti-node,” which is part of the standing wave that presents a high light intensity, exponentially attenuates. This manner of attenuation depends on the difference in the refraction index between multiple layers which form part of the DBR, and a larger difference in the refraction index results in faster attenuation and stronger confinement of light in the growth direction. Also, in the optical resonator unit, the light intensity exhibits a constant standing wave distribution substantially without attenuation.
In this way, the distribution of light in the growth direction depends on the penetration depth of light into the DBR and on the thickness of the optical resonator unit. For improving the current modulation efficiency at fr, it is effective to reduce the penetration depth of light into the DBR and the thickness of the optical resonator unit as much as possible.
The thickness of the optical resonator unit cannot take an arbitrary value since it must form a standing wave together with the upper and lower DBRs. Generally, an effective optical path length of an optical resonator unit is an integer multiple of λ/2, where λ is the oscillation wavelength of a VCSEL. Accordingly, a structure is provided to minimize the thickness of an optical resonator unit when the optical resonator unit has an effective optical path length equal to λ/2, and such a small optical resonator is known as a micro-resonator (micro-cavity) structure. The micro-cavity structure has the effect of inhibiting spontaneously emitted light other than the oscillation wavelength, so that the oscillation threshold can be lower to enable a greater reduction of the heat generated by the VCSEL at the same current value.
Techniques related to the structure of this λ/2 resonator are disclosed in JP-05-211346A, JP-07-193325A, and JP-10-256665A. Also, this technique is disclosed in a document (D. G. Deppe et al., Photonic Technology Letters, 1995, Vol. 7, No. 9, pp. 965-967).
In any of the structures disclosed in these documents, a λ/2 resonator is sandwiched on both sides by layers which have refraction indexes higher than the effective refraction index of the resonator unit. In this way, the anti-node of a standing wave stands substantially at the center of the resonator unit, and by placing an active layer here, a surface emitting laser can be formed to have a micro-cavity structure, as disclosed by the aforementioned documents. In the following, the surface emitting laser of this structure is referred to as a “λ/2 micro-cavity surface emitting laser.”
Here, a brief description will be given of a λ/2 micro-cavity surface emitting laser which is an example of a half-wavelength resonator among micro-cavities.
FIG. 1 is a diagram showing a band structure of the structure of a λ/2 micro-cavity surface emitting laser, and an electric field strength curve. FIG. 1 schematically represents a position in a growth direction (optical axis direction) on the horizontal axis, and band gap energy of each layer on the vertical axis. Assume that the electric field strength means a square of an absolute value of an electric field vector.
Thus, on the electric field strength curve, the “anti-nodes” present maximum values at peak positions of peaks and troughs of a standing wave before the electric field vector is squared, and “nodes” of the standing wave present minimum values. The maximum value as used herein includes the largest value. In the following, the “anti-node” refers to a point of the electric field strength curve at which the curve reaches a local maximum value, and the “node” refers to a point at which the curve reaches a local minimum value.
As shown in FIG. 1, optical resonator unit 212 is sandwiched between first multilayer Bragg reflecting mirror 211 and second multilayer Bragg reflecting mirror 213. In optical resonator unit 212, active layer 2121 is formed within a layer which has a large band gap (corresponding to a low refraction index layer) 2123. First multilayer Bragg reflecting mirror 211 comprises a plurality of laminated pairs of first low refraction index layer 2111 and first high refraction index layer 2112.
Second multilayer Bragg reflecting mirror 213 comprises a plurality of laminated pairs of second low refraction index layer 2132 and second high refraction index layer 2131. First high refraction index layer 2112 and second high refraction index layer 2131 are in contact with optical resonator unit 212. Referring to electric field strength curve 2122 indicative of the electric field strength of optical resonator unit 212, the anti-node of electric field strength curve 2122 is positioned at active layer 212.